This invention relates to metallacarboranes and to the use of metallacarboranes in the a of catalysis.
A carborane is a mixed hydride of boron and carbon containing a polyhedral framework of boron atoms which also includes one or more carbon atom. There are several classes of carboranes, two of which are of particular importance, the closo and nido categories. The prefix closo is used to designate those carboranes in which the framework is a complete deltahedron. The prefix nido designates those frameworks which are xe2x80x9copenxe2x80x9d, that is, one deltahedral site is incomplete.
Polyhedral closocarboranes have the formula Bnxe2x88x922C2Hn(nxe2x89xa75) and they may be prepared in a variety of ways. By way of examples reference will be made to dicarba-coso-dodecaboranes. 1,2-, 1,7- and 1,12-isomers exist, known respectively as ortho-caborane, meta-carborane and para-carborane. Icosahedral carboranes have high thermal stabilities, they are air stable and exhibit considerable chemical robustness.
Ortho-carboranes, R2C2B10H10, may be prepared by reaction of decaborane adducts, B10H12L2 (where L=dms, acetonitrile), with the appropriately substituted acetylene (RCxe2x95x90CR1).
Meta-carborane can be prepared quantitatively by thermal isomerisation of ortho-carborane at 460xc2x0 C., while para-carborane may be obtained by thermal isomerisation of the meta-isomer at 620xc2x0 C. However, the high temperatures required for these isomerisations render them unsuitable as routes to most substituted carboranes and alternative routes to substituted meta- and para-carboranes are used.
The hydrogen atoms on the carbons of dicarba-closo-dodecaboranes are acidic and thus undergo reaction with organolithium reagents (such as butyllithium) to produce C-lithiated carboranes. This carboranyl carbon lithium bond is itself susceptible to electrophilic attack, providing a means of preparing substituted meta- and para-carboranes. Carboranyl Grignard agents and copper derivatives can be prepared and used in a similar manner.
Other sigma-bonded carborane metal complexes have been reported in the literature, especially complexes of transition metals. F-block metal complexes have been reported but only with lanthanum, terbium and ytterbium as well as tentative reports of complexes with samarium and europium. No sigma-bonded actinide systems have been reported. Both 1,2- and 1,7-B10C2H12 can be degraded by strong bases to give isomeric B9C2H12 ions. This removal of a BH2+ unit from the parent carborane can be considered to result from nucleophilic attack at the most electron-deficient boron atom of the carborane. The B9C2Hxe2x88x9212 ions can be protonated to form the neutral nidocarborane, B9C2H13, which is a strong acid.
There are also many literature reports of pi-bonded metallacarboranes, again with transition metals. An actinide complex is reported by Fronczek et al in JACS, 99:6 1977 at page 1769. This results from the reaction of UCl4 with the B9C2H112xe2x88x92 anion in tetrahydrofuran solution and inert atmosphere conditions. The complex anion [U(B9C2H11)2Cl2]2xe2x88x92 has been isolated as several air-sensitive crystalline salts.
An NR2 bridged system has been reported by Plesek in Collect. Czech Chem Comm, vol 59, 1994 at page 374. An asymmetric cobaltacarborane complex is described which contains a monoatomic nitrogen bridge between both carborane units.
In addition to the carborane monomers mentioned above, substituted monomers may be prepared, an example being as shown in accompanying FIG. 9A.
Furthermore, oligomer structures have been prepared. By way of example, a structure having two carborane units is illustrated in FIG. 9B.
In addition, cyclic structures are known. Cyclic oligomers include dimer and trimer systems in particular, but higher ring sizes may be possible. The compounds shown in FIGS. 9C (where X is CH or N) and FIG. 9D (where X is Si, Sn or As) represent examples of cyclic structures.
In general, metals can be sandwiched between two nido anions or linked to just one anion to produce an xe2x80x9copen sandwichxe2x80x9d. An open sandwich structure leaves the remainder of the metal coordination available for occupation by other ligands. For example, certain RH(III) clusters have been disclosed by M F Hawthorne in Mol. Struct.Energ., vol 5, 1988, page 225 as homogeneous catalyst precursors for alkene isomerization or hydrogenation.
According to the present invention there is provided a polynuclear compound comprising two or more metal-hapto-3-capped nidocarborane groups. Preferably, a compound of the invention has at least one other real coordination site filled by a monodentate ligand.
Preferably, the compound of the invention has a cyclic structure.
Preferably, a compound of the invention includes two or three nidocarborane groups.
One class of compounds comprises oligomers of the polynuclear compounds. ie a chain of two or more of the polynuclear compounds of the invention, suitably linked by the reaction together of chemically reactive groups on subsituents of the nidocarborane units.
A preferred compound in accordance with the present invention is one in which two or more nidocarborane groups are linked together by a phenyl, pyridyl, triazyl, B(OH), CO, SO, CH2 or B(Ph) group, such that linear or cyclic structures are formed. The cyclic structures may have metals coordinated in the central cavity.
Preferred compounds contain a metal taken from F block metals, Rh, Co, Mn, Ru, Pd, W, Fe, Ni, Pt, Cr or Mo, and may contain the same metal at each site or different metals at each site.
A preferred metallacarborane includes two or more carborane units linked by means of a benzene ring, a pyridine ring or a triazine ring.
Preferably, the carborane units are functionalised (ie substituted) or linked in the 1,3, 1,4 or 1,5 (ie ortho, meta or para) positions. For functionalised carborane units, various substituent groups can be used including phenyl, alkyl (eg methyl) or a group (eg containing a phenyl or alkyl residue) having a reactive factional group (eg xe2x80x94COOH, an ester or an unsaturated group such as an ethylenic double bond). The functionalisation may or may not be symmetrical.
For the linkages within the linear or cyclic polynuclear compounds, particularly preferred units include phenyl (connected meta or para), pyridine, CO, SO, CH2 and B(OH).
The present invention also provides a method of carrying out a chemical reaction in which a compound of any of the preceding claims acts as a catalyst in the reaction. Compounds of the invention may be of use in, for instance, hydrogenation, isomerisation, polymerisation and oxidation reactions. For instance, a compound of the invention in which the metal is Rhodium may be used as a hydrogenation catalyst and a compound of the invention in which the metal is Uranium may be used as an oxidation catalyst.